A spaghettiverse (or knotted verse) is a type of universe or other cosmological structure that allows for travel between its own Everett branches. So named because of their intersecting, tangled timelines, in sharp contrast with the clean causal trees of most universes, spaghettiverses are extremely dangerous, becoming more chaotic over time and rapidly deteriorating even further.
The name applies both to the individual universes corresponding to specific branches or to the multiverse that contains such entangled universes or even to any higher verse as long as the verses it contains are all entangled in a similar manner. As long as the verses directly contained by a higher verse are all entangled in such a way, that higher verse can be considered a spaghettiverse.
Dangerously increasing complexity[]
Forming a spaghettiverse is rather simple; it involves starting with a type I-a altverse - a multiverse consisting of one universe's possible branches - and adding the condition that travel between each of these causal paths is possible. Keeping said spaghettiverse stable, on the other hand, is incredibly difficult.
Whenever an entity or effect travels from one branch to another, it starts another chain of branches where it arrived very slightly differently and with very slightly different consequences, and thus creates an immense number of instances of itself tunneling out to new branches in the process. Every time there is a possibility that some entity, object or even particle will jump to some other branch, multiple new branches will be created just from that possibility alone. At any point in time that something can jump to some other branch, that original branch will split in two for each thing that could jump, one where it jumps and one where it doesn't. If the jumper is a composed object, for example, if it is made of multiple smaller particles, there will be multiple forks based on what exact particles where part of the larger object and in what state did it jump.
New forks also occur on every branch where a jumper could land, one where it lands and one where it doesn't, and even more forks for all the possible ways and moments in which it could land from the jump. This means that for a single fork of the original branch where the jumper disappeared by jumping into another branch, new forks are created on all the possible destination branches where the jumper could land, most likely multiple forks on each for all the different possible ways on which it could land.
It is also perfectly possible for someone to jump to another branch where they already exist or for multiple versions of the same being to all jump to the same branch and meet that way. What if the multiple versions of the same jumper try to jump back to their original branch, they will all meet there and there is no way of determining the real original one because they all will be the real original one. And others still would have stayed behind on forks of the new branches where they do not try to jump back to the original one, and others still where they will jump but to yet other branches and so on. These exponentially-replicating groups quickly overwhelm vast numbers of timelines and render large swathes of possibility uninhabitable.
Complexity increases at higher verses[]
Spaghettiverses get even worse at larger scales. The state of a multiverse will include all the existing branches of its contained universes and when it forks, all of those are forked. Every single branch of the multiverse will contain every branch of all the contained universes, possibly in different configurations, which is what accounts for the existence of different multiverse level branches in the first place. Usually jumping from one multiverse level branch to another will leave one in the same position at the universe level branches but if it is also possible to jump from one universe level branch to a different universe level branch that is located in a different multiverse level branch than the increase in complexity will be dramatic.
For example, imagine a scenario in which an entity in one branch of a spaghettiverse itself composed of spaghettiverses is imprisoned. Since a "macro-scale" branch exists where a rescuer passes through a portal to this specific branch and retrieves them, it must occur at some point, and in the local macro-scale branch, which isn't otherwise specified, it can occur at any point.
Compounding this on every relevant scale leads to the conclusion that effectively anything can happen in a large enough spaghettiverse, at any time, for any reason. Every entity is a reality-warper in a sense, but none have any actual control; even direct interference by local gods (albeit those not dimensionally extensive enough to bypass the timeline-splitting effects in the first place) can create new chains of branches as the alternate gods attempt to fix the situation in their own ways. As a result, the more powerful beings of the omniverse prefer to leave these -verses alone, letting them explode into a chaotic riot of complete absurdity before succumbing to total reality collapse or the like.